Kevin Müller

Oil-in-oil emulsions: A unique tool for the formation of polymer nanoparticles

Polymer latex particles are nanofunctional materials with widespread applications including electronics, pharmaceuticals, photonics, cosmetics, and coatings. These materials are typically prepared using waterborne heterogeneous systems such as emulsion, miniemulsion, and suspension polymerization. However, all of these processes are limited to water-stable catalysts and monomers mainly polymerizable via radical polymerization. In this Account, we describe a method to overcome this limitation: nonaqueous emulsions can serve as a versatile tool for the synthesis of new types of polymer nanoparticles. To form these emulsions, we first needed to find two nonmiscible nonpolar/polar aprotic organic solvents. We used solvent mixtures of either DMF or acetonitrile in alkanes and carefully designed amphiphilic block and statistical copolymers, such as polyisoprene- b-poly(methyl methacrylate) (PI- b-PMMA), as additives to stabilize these emulsions. Unlike aqueous emulsions, these new emulsion systems allowed the use of water-sensitive monomers and catalysts. Although polyaddition and polycondensation reactions usually lead to a large number of side products and only to oligomers in the aqueous phase, these new conditions resulted in high-molecular-weight, defect-free polymers. Furthermore, conducting nanoparticles were produced by the iron(III)-induced synthesis of poly(ethylenedioxythiophene) (PEDOT) in an emulsion of acetonitrile in cyclohexane. Because metallocenes are sensitive to nitrile and carbonyl groups, the acetonitrile and DMF emulsions were not suitable for carrying out metallocene-catalyzed olefin polymerization. Instead, we developed a second system, which consists of alkanes dispersed in perfluoroalkanes. In this case, we designed a new amphipolar polymeric emulsifier with fluorous and aliphatic side chains to stabilize the emulsions. Such heterogeneous mixtures facilitated the catalytic polymerization of ethylene or propylene to give spherical nanoparticles of high molecular weight polyolefins. These nonaqueous systems also allow for the combination of different polymerization techniques to obtain complex architectures such as core-shell structures. Previously, such structures primarily used vinylic monomers, which greatly limited the number of polymer combinations. We have demonstrated how nonaqueous emulsions allow the use of a broad variety of hydrolyzable monomers and sensitive catalysts to yield polyester, polyurethane, polyamide, conducting polymers, and polyolefin latex particles in one step under ambient reaction conditions. This nonpolar emulsion strategy dramatically increases the chemical palette of polymers that can form nanoparticles via emulsion polymerization.

PROPAGATION AND DEFLECTION OF GUIDED MODES IN PLANAR WAVE-GUIDES VIA GRATING ROTATION

Propagation and deflection of guided modes in a polymer slab waveguide is studied. Rotation of the grating structure with respect to the plane of incidence deflects the waveguide modes in the plane of the sample so that the propagation direction is no longer perpendicular to the grating grooves. The deflection angles are measured as a function of the grating rotation angle and the experimental results are compared with theoretically expected values. The difference in polarization behaviour between waveguide and plasmon surface polariton grating rotation coupling is shown.

Evanescent (surface) wave holographic recording in thin polymeric film

Holographic grating recording with surface evanescent waves is reported. Owing to the small penetration depth the interference pattern is sorted in a 200 nm thick polystyrene film, sensitized with ON-stilbene. The absorption distribution in the recording medium is analysed in the small attenuation index approximation. The diffraction efficiency dependence on time and temperature is also investigated.

Thin azo-polymer films for reversible holographic recording

Polymethylmetacrylate with covalently bound side-chain azo- benzene group is synthesized and thin films are fabricated. Holographic grating recording is achieved with a 442 nm He- Cd laser at three different grating spacing: 646, 1340 and 2220 nm. The dependence of the diffraction efficiency on time is investigated. The optical recording is stable for more than four years at room temperature. Optical erasure can be performed with circularly and linearly polarized laser light.